1. Field of the Invention
The present invention relates to a composition for forming an insulating film. More particularly, the invention relates to a composition for forming an insulating film which is capable of forming a coating film having an even thickness and suitable for use as an interlayer insulation film in semiconductor elements or the like, and an insulating film produced from the composition which is less apt to crack and excellent in dielectric constant characteristics, etc.
2. Description of the Related Art
Silica (SiO2) films formed by vacuum processes including the vapor-phase epitaxy (CVD) process have been in frequent use as interlayer insulation film in semiconductor elements and the like. In recent years, an insulating film called an SOG (spin-on-Glass) film, which is formed from a coating fluid and comprises a tetraalkoxysilane hydrolysate as the main component, has come to be used for the purpose of forming a more even interlayer insulation film. Furthermore, an interlayer insulation film called an organic SOG film, which has a low dielectric constant and comprises a polyorganosiloxane as the main component, has been developed as a result of progress in the degree of integration in semiconductor elements and the like.
However, even the CVD-SiO2 film, which is the lowest in dielectric constant among the inorganic material films, has a dielectric constant of about 4. Furthermore, although the SiOF film, which is being recently investigated as a low-dielectric constant CVD film, has a dielectric constant of about 3.3 to 3.5, this film has a problem that it is highly hygroscopic property and the dielectric constant thereof increases during use.
On the other hand, organic polymer films having a dielectric constant as low as 2.5 to 3.0 have a glass transition temperature as low as 200 to 350° C. and further have a high coefficient of thermal expansion. Because of this, the organic polymer films have a problem concerning damage to wirings. Moreover, the organic SOG film has a drawback that it suffers oxidation by the oxygen plasma ashing used for, e.g., resist removal in multilayer wiring pattern formation, and cracks.
In addition, the organic resins including the organic SOG film have the following problems. Since they have low adhesion to wiring materials, i.e., aluminum, aluminum-based alloys, copper, and copper-based alloys, application thereof results in voids beside the wiring (gaps formed between the wiring and the insulating material). There is a possibility that water might penetrate into the voids to cause wiring corrosion. Moreover, these voids generated beside wirings lead to short-circuiting between wirings when positional shifting occurs during the formation of via holes for forming a multilayered wiring, resulting in reduced reliability.
Under these circumstances, a coating composition for forming an insulating film comprising a copolymer containing an organopolysiloxane having a cage structure, specifically, a hydrogenated octasilsesquioxane, is known as an insulating-film material excellent in insulating properties, heat resistance, and durability (JP-A-11-40554).
However, with the trend toward a higher degree of integration and a higher degree of multilayer constitution in semiconductor elements and the like, better electrical insulation between conductors has come to be required. Namely, an interlayer insulation film material having a lower dielectric constant and excellent in crack resistance, heat resistance, and resistance to chemical mechanical polishing (CMP) has come to be desired.